This invention generally relates to the spin application of photoresist, polyimide, and other liquid materials. More particularly, it relates to an apparatus and method for spin applying the liquid materials to a planar top surface of an electronic substrate so that the edge and back of the substrate are not coated with the material.
Liquid coating materials such as photoresist and polyimide are most often applied to electronic substrates such as semiconducting wafers, chip-carrying substrates, and printed circuit boards by methods including spin, spray, and dip. These materials are used for a variety of purposes including photolithography, mask layers and electrical insulators. A spin application is preferred for these materials in semiconductor processing because it generally provides a thin, uniform coating.
However, there are at least two major problems encountered during spin apply process. The first problem is that a thick layer of material known as an edge bead usually forms near the substrate edge. The edge bead is the result of surface tension within the film that draws the material in from the substrate edge. Because the thickened material does not cure as fully as the thinner coating on the rest of the surface, it degrades during further processing and is a source of particulate contamination. If the thicker material is photoresist, lithography processing is generally impossible in this area. The edge bead is therefore a concern for integrated circuits and high density substrates. For semiconductor wafers, this excess thickness of resist at the edge of the wafer is responsible for a higher defect level at the wafer edge, lower yield, and lower reliability for chips located near the edge of the wafer.
The second problem is that the coating material often wets the wafer edge and creeps around to the back of the wafer during the spinning. Material in these locations is also a source of particulate contamination. Photoresist on the wafer back can prevent proper leveling and focusing of the lithography tool during a subsequent photoexposure step.
Thus, in conventional processing, the edge bead and the material on a wafer edge and back must be removed before further processing. Typically, the removal is accomplished by a chemical spray directed at the wafer before the resist is baked. However, it is known that resist dissolving chemicals cause bulging of the resist adjacent to the removed resist. This degraded resist is also a source of particulate contamination, especially if the wafer is subjected to an ultraviolet hardening step. For this reason, an additional segment of edge resist, that exposed to the resist removing chemical, must also be removed. This additional segment of resist is removed by exposing and developing the edge region. The two removal steps cause 1 to 2 mm of resist to be lost from for the outer edge of the top surface of the wafer.
U.S. Pat. No. 4,086,870 describes an edge masking technique using a knife edge cover plate mask which prevents resist from coating an outer ring of the wafer. As resist is usually much thinner than a knife edge, the knife edge appears as a wall impeding the free flow of material off the spinning wafer, causing non-uniformity in thickness and splashes of particles which land back on the wafer. Furthermore, capillary action forces resist under the knife edge cover plate allowing the resist to wet the wafer edge and back. Variations in wafer thickness and irregularities in the wafer surface or in the thin knife edge surface provide gaps, increasing the amount of leaking under the mask. Also, the top cover plate covers part of the active area of the wafer. The loss of semiconductor real estate due to the knife edge becomes more important as the industry moves to larger wafer sizes, thus losing a greater number of potential chip sites at the edge of the wafer. Finally, the complicated mechanism makes automation difficult.